The proposed research program will complete the MLR and CML assays of 30 cadaveric renal transplant recipients given TLI and horse or rabbit ATG, and who have good graft function while receiving only low dose prednisone. In each case specific unresponsiveness to donor cells will be tested. On the basis of our preliminary studies, we expect about 80% of patients to show this pattern. In order to determine whether specific unresponsiveness in the MLR is due to clonal deletion and/or active suppression, purified helper T cells (Leu-3+) will be tested against donor and third-party cells in the MLR. If the latter cells show specific unresponsiveness, then this would provide evidence for clonal deletion, since suppressor/cytotoxic cells (Leu-2+) will have been removed. If the purified helper T cells show reactivity to the donor cells with or without the addition of IL-2 and this is lost after the add-back of purified Leu-2+ cells, then this would provide evidence for active suppression. In addition, we will try to identify the specific Class II HLA antigens to which unresponsiveness is directed so that purified forms of this material may be used in the latter protocols as potential tolerogens. In similar studies, we propose to determine whether specific unresponsiveness in the CML assay is due to a lack or reactivity to donor cells by the helper T cell subset alone or in combination with the suppressor/cytotoxic T cell subset (Leu-2+ cells). pCTL cells (Leu-2+, 9.3+) will be incubated in the presence of IL-2 with donor and third-party stimulator cells in order to generate cytolytic cells. If the purified pCTL cells show specific unresponsiveness to the donor cells, then this will provide evidence for clonal deletion. On the other hand, if pCTL show reactivity to donor cells, and are specifically suppressed by the addition of purified suppressor cells (Leu-2+, 9.3-), then we will have obtained evidence for active suppression. If suppressor cells of the MLR or CML are detected, then we will attempt to establish clone lines of the donor specific suppressor cells using previously described techniques for the development of human alloantigen specific suppressor cell lines. A critical hypothesis to be tested is that the development of specific unresponsiveness in the MLR and CML predict the ability to completely withdraw immunosuppressive drugs.